1. Field of the Invention
The present invention relates to a scanning optical apparatus, an image forming apparatus, and an image clock correction method that is applied to a scanning optical apparatus.
2. Description of the Related Art
A scanning optical apparatus that is utilized with an image forming apparatus or the like scans a beam along a main scanning direction. The main scanning direction is the axial direction (lengthwise direction) of a photosensitive member. Particularly in an electrophotographic image forming apparatus, a scanning optical apparatus modulates a beam with an image signal, and a polygonal mirror, which is driven by a motor, rotates and deflects the modulated beam to scan a scanning plane on a photosensitive member.
In this kind of image forming apparatus, it is desirable that the magnification ratio in the main scanning direction is always an ideal magnification ratio. However, a so-called “shift of variable magnification ratio” occurs due to a mounting error when mounting a scanning optical apparatus onto an image forming apparatus, limits of accuracy of the instrument in use (refractive index fluctuations) or errors when assembling an optical component such as a reflection mirror or an f-θ lens. A shift of the variable magnification ratio is undesirable because it becomes a cause of image distortion. In particular, a color image forming apparatus forms a color image by multiple-transferring toner images of each color on a transfer member. Therefore, if the amount of a shift of the variable magnification ratio is different for each color, it becomes a cause of color misalignment. As a results, the image quality deteriorates undesirably.
Japanese Patent Laid-Open No. 2005-111972 discloses a technique of dividing the main scanning direction in a plurality of areas, adjusting the position of the image clock for each area, and correcting the magnification ratio components of the areas which differ from one another due to optical components or the like.